1. Field of the Invention
The present invention relates to a projection system, and more particularly, to a highly efficient projection system which can be made more compact by using a single scrolling unit to scroll color bars and more light efficient by effectively utilizing polarization components.
2. Description of the Related Art
In general projection systems, a light valve, such as a liquid crystal display (LCD) or a Digital Micro-mirror Device (DMD), controls the on/off operation of light emitted from a light source on a pixel-by-pixel basis and forms a picture. A magnifying projection optical system provides the picture to a large screen.
Projection systems are classified into either 3-panel projection systems or single-panel projection systems, according to the number of light valves used. The 3-panel projection systems provide better optical efficiency than the single-panel projection systems, but are generally more complicated and expensive. The single-panel projection systems can have a smaller optical system than the three-panel projection systems. However, these single-panel systems provide only ⅓ of the optical efficiency of the three-panel projection systems because red (R), green (G), and blue (B) colors into which white light is separated are used sequentially. To be more specific, in a single-panel projection system, white light radiated from a white light source is separated into R, G, and B color beams using color filters, and the three color beams are sequentially transmitted to a light valve. The light valve operates according to the sequence of color beams received and creates images. As described above, a single-panel projection system uses color beams sequentially, therefore, the light efficiency is reduced to ⅓ the light efficiency of a three-panel projection system.
According to one color scrolling method designed to increase the optical efficiency of a single-panel projection system, white light is separated into R, G, and B color beams, and the three color beams are simultaneously sent to different locations on a light valve. Since an image cannot be produced until each of the R, G, and B color beams reach all pixels of the color areas in the light valve, the color beams are moved at a constant speed by a color scrolling means.
FIG. 1 shows a single-panel scrolling projection system disclosed as in U.S. Publication No. 2002/191154 A1. As shown in FIG. 1, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104, a polarization conversion system (PCS) 105, and a condenser lens 107, and is separated into R, G, and B color beams by first through fourth dichroic filters 109, 112, 122, and 139. To be more specific, the red beam R and the green beam G, for example, are transmitted by the first dichroic filter 109 and advance along a first light path L1, while the blue beam B is reflected by the first dichroic filter 109 and travels along a second light path L2. The red beam R and the green beam G on the first light path L1 are separated by the second dichroic filter 112. The second dichroic filter 112 transmits the red beam R along the first light path L1 and reflects the green beam G along a third light path L3.
The light emitted from the light source 100 is separated into the red beam R, the green beam G, and the blue beam B, and they are then scrolled while passing through corresponding first through third prisms 114, 135, and 142. The first through third prisms 114, 135 and 142 are disposed on the first through third light paths L1, L2, and L3, respectively, and rotate at a uniform speed such that R, G, and B color bars are scrolled. The green beam G and the blue beam B that travel along the second and third light paths L2 and L3, respectively, are transmitted and reflected by the third dichroic filter 139, respectively, and then combined. Finally, the R, G, and B beams are combined by the fourth dichroic filter 122. The combined beam is transmitted by a polarization beam splitter (PBS) 127, which forms a picture using a light valve 130.
The scrolling of the R, G, and B color bars due to rotation of the first through third prisms 114, 135, and 142 is shown in FIG. 2. Scrolling represents the movement of color bars formed on the surface of the light valve 130 when the first, second, and third prisms 114, 135, and 142 corresponding to colors are synchronously rotated.
A color image obtained by turning on or off the individual pixels of the light valve 130 according to an image signal is magnified by a projection lens (not shown). Then, the magnified image is made incident on a screen.
Since the conventional projection system uses different light paths for each color as described above, different light path correction lenses must be included for each color, component parts for recollecting separated light beams must be further included, and separate component parts must be included for each color. Hence, the optical system becomes bulky, and the manufacture and assembly thereof is complicated, thus degrading the yield.
Three motors for rotating three scrolling prisms for three color beams generate a lot of noise during operation. Additionally, a projection system utilizing three motors is manufactured at a greater cost than a color wheel type projection system which utilizes a single motor.
In order to produce a color picture using a scrolling technique, color bars as shown in FIG. 2 must be moved at a constant speed. The conventional projection system must synchronize the light valve 130 with the three prisms 114, 135, and 142 in order to achieve scrolling. However, controlling the synchronization is not easy. Due to the circular motion of the scrolling prisms 114, 135, and 142, the color scrolling speed by the three scrolling prisms is irregular, consequently deteriorating the quality of the resultant image.
FIG. 3 is a magnification of the PCS 105 of FIG. 1. Referring to FIG. 3, an unpolarized beam emitted from the light source 100 of FIG. 1 passes through the second lens array 104 and is then incident upon the PCS 105. The PCS 105 converts an incident beam into a beam with a single polarization. The PCS includes a PBS array 123 and a ½ wavelength plate 122, which is installed adjacent to the PBS array 23 and which changes a polarization direction. In this structure, a first beam with a P polarization, among P and S polarizations of an unpolarized beam received from the second lens array 104, is transmitted by the PBS array 123, and a second beam with an S polarization is reflected by a mirror 123afor reflecting an S polarization. Thereafter, the second beam with the S polarization is re-reflected by the mirror 123a and then advanced in the same direction as the first beam with the P polarization. The second beam with an S polarization, oriented in the same direction as the first beam, is converted into a beam with a P polarization while passing through the ½ wavelength plate 122.
As described above, the PCS 105 increases the optical efficiency by converting an unpolarized incident beam into a beam with a single uniform polarization. However, the PCS 105 having the above-described structure generates a beam loss because of cell boundaries of the second lens array 104. Further, this structure is complicated.